<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8" />
  <meta name="viewport"
    content="maximum-scale=1.0,minimum-scale=1.0,user-scalable=0,width=device-width,initial-scale=1.0" />
  <title>timeline</title>
  <style>
    .main {
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
    }

    .container {
      display: flex;
    }

    .mod {
      display: flex;
      margin: 10px;
      width: 120px;
      height: 120px;
      border: 2px dashed #000;
    }

    .center-h-v {
      margin: auto;
    }

    /*实心右箭头*/
    .to_solid_right {
      width: 0;
      height: 0;
      border-style: solid;
      border-width: 10px 0 10px 20px;
      border-color: transparent transparent transparent #000;
    }

    /*实心左箭头*/
    .to_solid_left {
      width: 0;
      height: 0;
      border-style: solid;
      border-width: 10px 20px 10px 0;
      border-color: transparent #000 transparent transparent;
    }

    /*实心上箭头*/
    .to_solid_top {
      width: 0;
      height: 0;
      border-style: solid;
      border-width: 0 10px 20px 10px;
      border-color: transparent transparent #000 transparent;
    }

    /*实心下箭头*/
    .to_solid_botton {
      width: 0;
      height: 0;
      border-style: solid;
      border-width: 20px 10px 0 10px;
      border-color: #000 transparent transparent transparent;
    }

    /*空心下箭头*/
    .to_botton {
      width: 10px;
      height: 10px;
      transform: rotate(45deg);
      border-bottom: 1px solid #000;
      border-right: 1px solid #000;
    }

    /*空心上箭头*/
    .to_top {
      width: 10px;
      height: 10px;
      transform: rotate(315deg);
      border-top: 1px solid #000;
      border-left: 1px solid #000;
    }

    /*空心右箭头*/
    .to_right {
      width: 10px;
      height: 10px;
      transform: rotate(45deg);
      border-right: 1px solid #000;
      border-top: 1px solid #000;
    }

    /*空心左箭头*/
    .to_left {
      width: 10px;
      height: 10px;
      transform: rotate(45deg);
      border-left: 1px solid #000;
      border-bottom: 1px solid #000;
    }
  </style>
</head>

<body>
  <div class="main">
    <div class="container">
      <div class="mod">
        <div class="to_top center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_botton center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_left center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_right center-h-v"></div>
      </div>
    </div>
    <div class="container">
      <div class="mod">
        <div class="to_solid_top center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_solid_botton center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_solid_left center-h-v"></div>
      </div>
      <div class="mod">
        <div class="to_solid_right center-h-v"></div>
      </div>
    </div>
  </div>
</body>

</html>